Energy recovery systems have been proposed for recovering surplus energy produced during operation of various kinds of working devices to improve the energy efficiency of the working devices. Such an energy recovery system converts heat generated during operation of the working devices into electrical energy. A typical energy recovery system has a thermoelectric converter that generates electricity by utilizing a temperature difference between a high temperature thermal medium (high temperature medium) and a low temperature thermal medium (low temperature medium). As a high temperature medium, a thermal medium that receives heat during operation of the working devices is used, and as a low temperature medium, a thermal medium the temperature of which is lower than the high temperature medium is used. The thermoelectric converter uses waste heat from the working devices to generate electricity. In other words, heat is converted into electrical energy.
Such energy recovery systems include types that are applied to vehicles such as automobiles. For example, Japanese Laid-Open Patent Publication No. 2001-23666 discloses an energy recovery system for an automobile. When the power source and other working devices of a vehicle are operating, heat from the power source and other working devices escapes as waste heat. The system disclosed in the publication uses the waste heat to generate electricity in a thermoelectric converter, which electricity is used for charging the vehicle battery. In this way, the waste heat from the power source and other working devices is recovered as electrical energy. In the energy recovery system disclosed in the publication, the thermoelectric converter uses the coolant for cooling the power source and other working devices as a high temperature medium, and uses the outside air, the temperature of which is lower than that of the coolant, as a low temperature medium.
However, in a vehicle using an energy recovery system according to the above publication, normal operation of the power source and other working devices cannot increase the temperature of the power source and other working devices (the temperature of the coolant) above a certain level. Therefore, the temperature difference between the coolant and the outside air can be insufficient for generating electricity with the thermoelectric converter. Therefore, in Japanese Laid-Open Patent Publication No. 2001-23666, operation of the power source and other working devices is controlled to increase the coolant temperature so that the temperature difference between the coolant and the outside air is sufficient for generating electricity. However, the control for increasing the temperature of the power source and other working devices can adversely affect the power source and other working devices.
To avoid such drawbacks, Japanese Laid-Open Patent Publication No. 2002-59736 discloses a technique applied to a case where an internal combustion engine is used as a drive source. To generate electricity with a thermoelectric converter, the technique uses the temperature difference between exhaust of the engine and the coolant temperature, instead of the temperature difference between the coolant and the outside air. In this case, the low temperature medium is the coolant and the high temperature medium is the exhaust. Since the temperature of the exhaust is significantly higher than the coolant temperature, normal operation of the engine guarantees a sufficient temperature difference for generating electricity through thermoelectric conversion. However, although the temperature of the coolant is relatively stable, the temperature of the exhaust greatly varies depending on the operating state of the engine, for example, in a range from 100° C. to 800° C. Such great temperature changes can damage the thermoelectric converter.
Thus, to reliably generate electricity with a thermoelectric converter that utilizes a temperature difference, it is important that the temperature difference be sufficiently great and that the temperatures of the used thermal media be stable. Therefore, it has been proposed that the high temperature medium be coolant from the power source and other working devices and that the low temperature medium be a low temperature refrigerant since the temperature of coolant is relatively stable, and the temperature of a refrigerant is forcibly maintained low by a refrigeration apparatus. In this case, although coolant from the power source and other working devices is used as the high temperature medium, the temperature difference between the high temperature medium and the low temperature medium is sufficient for generating electricity since the low temperature refrigerant, which is the low temperature medium, is maintained to be cold by the refrigeration apparatus. Further, since the temperature of the low temperature refrigerant is maintained in a certain range by the refrigeration apparatus, the temperature of the refrigerant is not abruptly changed, which prevents the thermoelectric converter from being damaged.
Therefore, using low temperature refrigerant as the low temperature medium, the temperature of which is maintained to be low by a refrigeration apparatus, permits the temperature difference between the high temperature medium and the low temperature medium to be sufficient for generating electricity, and prevents the thermoelectric converter from being damaged. Accordingly, the thermoelectric converter is capable of reliably generating electricity.
However, if a great amount of energy is consumed to drive the refrigeration apparatus so that the low temperature refrigerant is maintained to be cold, a possible improvement of the energy efficiency, which would be brought about by the energy recovery system, is hindered. Such problems occur not only in a case where the energy recovery system is used in a vehicle such as an automobile, but also in a case where the energy recovery system is applied to various apparatuses other than vehicles.